Background noise reducing circuit for audio frequency translating circuit



H. TUNICK QUENCY TRANSLATING CIRCUITS Original Filed March 9, 1940' FRE BACKGROUND NOISE REDUCING CIRCUIT FOR AUDIO Aug. 19,

, INVENQOR. BY wfimw Patented Aug. 19, 1947 BACKGROUND NOISE REDUCING CIRCUIT FOR AUDIO FREQUENCY TBANSLATING CIRCUIT Harry Tunick, Rye, N. Y., assignor to ItadioCorporation of America, a corporation of Delaware Original application 'March 9, 1940, Serial No.

Divided and this application March 5. Serial No. 585,089 r Claims. (Cl. 179100.4)

My present invention relates to a receiver for receiving and translating frequency modulated (FM) waves. This application is a division of my copending application Serial No. 323,089, filed March 9, 1940.

The principal object of my present invention is to provide an improved frequency modulation receiver having an improved signal to noise ratio.

Other objects of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the receiver shown in the drawing, FM waves are collected by antenna 2, and fed through tunable broad passband coupling 4 to radio frequency amplifier B. The output of radio frequency amplifier 6 is fed to the first detector 8, which is also supplied with oscillations from local oscillator l0. Beat frequency energy is fed into the intermediate frequency (I. F.) amplifier and limiter stages it. These are schematically represented. Radio frequency amplifier 6 is provided with a detector or rectifier I2, operating as an automatic volume controlling rectifier, whose rectified voltage output controls the gain of radio frequency amplifier 6 through a suitable resistorcondenser time constant circuit [4. If desired, the AVG detector input leads may be connected to the output leads of the first detector 8. In either event, the automatic volume control (AVC) leads 24 may be connected as shown, or they may be connected to amplifier 26 to vary the magnitude of the energy fed from oscillator I0 to the first detector. If desired, leads 24 may be connected to both radio frequency amplifier 6 and to amplifier 26 simultaneously to control the gain or output thereof. The purpose of having the energy fed to the limiters of the I. F. amplifier stages volume controlled is to prevent the limiters from being subjected to wide variations whereby the output of the I. F. limiter stages to the input side of FM detectors 30 and 34 is substantially constant, and devoid of drooping levels following from under, or over, excitation of limiters.

Circuits 32 and 36 are respectively tuned to opposite sides of the mean intermediate frequency, and have overlapping resonance curves. The outputs of detectors 30 and 34 are connected in phase opposition, and fed through transformer 38 to audio frequency amplifier 40 connected to a loudspeaker 42. The FM detector is of the type shown by Conrad in U. S. Patent No. 2,057,640.

For high fidelity reception audio frequency -ampl'ifieri't and loudspeaker 42- are-made substantiall -fiatfromabout 20 to 15,000 cyclesr Since the higher notes above 8,000 or 9,000 cycles to l5,000 cy'cles'v are present only in relatively infrequent programs such as presented by symphonyTo-rchestras;etc; Lprovide an arrangement to keep high frequency noise background out of the loudspeaker when the usual run of the mill programs are received.-

To-do-this I connect a high pass filter 44, which passes 8, 000to' 1 5,000 cycles only, across the input side of the audio. amplifier 40. Leeds 44' are the connections" fromthe input terminals of amplifier 40 to the filter input terminals. The output of filter 44 is fed to rectifier and D. C. amplifier 46 feeding the resistor-condenser time constant circuit 48. Bias voltage source 49 is chosen so that in the presence of only high frequency background noise which is of low level, tube 50 is substantially conductive. I-Iigh pass filter 54 is arranged freely to pass energy from 8,000 to 15,000 cycles, and, hence, effectively grounds the loudspeaker 42 for these noise frequencies through bypassing condenser 52 and normally-conducting tube 50. However, in the presence of musical frequencies strong enough to override the background noise, high pass filter #4 passes enough of such energy to rectifier 46 to bias the grid of tube 50 negatively to a higher degree, and thus causes tube 50 to become nonconductive. Hence, filter 54 is effectively opencircui-ted, and the higher audio frequencies are no longer shunted away from, but are permitted to flow into, loudspeaker 42. If desired, a switch 55 may be provided to render inoperative the input to filter 54.

It is to be clearly understood that the pass band of 8,000 cycles to 15,000 cycles is merely given by way of example. Other limiting values may be chosen. For example, the overall pass band for the audio frequency stages may be from 10 to 10,000 cycles. High frequency noise elimination in the absence of signal over-tones may be made to take place in the band from 7,000 cycles to 10,000 cycles, for example.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination, a channel for translating frequency signals, and means responsive to the intensity of the high audio frequency signals in said channel for controlling the attenuation of a said signals by said attenuating means.

2. In combination, in a frequency modulation receiver, a translating channel for audio frequency signals, means coupled to said channel for substantially removing audio signals having a frequency greater than a predetermined frequency from the output of said channel, and means including a rectifier having a filter input responsive to the appearance of audio signals of a frequency greater than said predetermined frequency in the input to said channel to disable said signal removing means.

3. In combination, a background noise reducing circuit for an audio frequency translating circuit comprising, a capacitance and a resistance connected in series across said audio circuit, said resistance being the plate to cathode impedance of an electron discharge device, said device having a control electrode, and said noise reducing circuit being adjusted to remove undesired high frequency currents from said audio circuit, and

means to disable said noise reducing circuit to permit translation of desired high frequency currents comprising, means to rectify high frequency voltagesrpresent in said audio circuit, and means for applyingsaid rectified voltages to said control electrode in a gain reducing sense.

4. In combination, a channel for translating audio frequency signals, means coupled to said channel for attenuating the high audio frequency signals therein while permittingtranslation of lower audio frequency signals, and means responsive to the presence of intense high audio frequency signals in said channel for reducing the attenuation of said signals by said attenuating means.

HARRY TUNICK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,784,839 Keller Dec. 16, 1930 2,207,934 Mountjoy July 16, 1940 2,369,952 Devine Feb. 20, 1945 

